560. Subarray Sum Equals K

Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals to k.

Example 1:

Input:nums = [1,1,1], k = 2
Output: 2

 

Note:

1. The length of the array is in range [1, 20,000].
2. The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

class Solution {
public:
    int subarraySum(vector<int>& nums, int k) {
        unordered_map<int,int>mp;
        int presum=0;
        mp[0] = 1;
        int res=0;
        for(int i = 0;i<nums.size();i++)
        {
            presum+=nums[i];
            if(mp.find(presum-k)!=mp.end())
                res+=mp[presum-k];
            mp[presum]++;
        }
        return res;
    }
};